Add power blackout error

A blackout is when both BMC power and main power are lost.  This error
can be used to notify a user that this occurred, after the fact of
course.

Change-Id: I8d84c9753f0eeb395a989758692dbe5109b36f3f
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/xyz/openbmc_project/State/Shutdown/Power.errors.yaml b/xyz/openbmc_project/State/Shutdown/Power.errors.yaml
index b364c2d..2786ae3 100644
--- a/xyz/openbmc_project/State/Shutdown/Power.errors.yaml
+++ b/xyz/openbmc_project/State/Shutdown/Power.errors.yaml
@@ -1,2 +1,5 @@
 - name: Fault
   description: The system was shut down because a power fault was detected.
+
+- name: Blackout
+  description: The system suffered a power blackout.